Choosing the Right Dealkalizer for Your System
Author: Claire Beaureagard | Business Development Manager
A dealkalizer is a pre-treatment device commonly seen in boiler rooms which is responsible for removing carbonate alkalinity in the feed water before it reaches the boiler. In many cases, the water chemistry parameter which hits its limit first is alkalinity. Thus, by reducing the alkalinity in the feedwater, we can decrease blowdown volume, increase the cycles of concentration, and effectively decrease energy consumption.
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The first step in boiler water treatment is to analyze the untreated raw water to determine the type and amount of impurities present. The solution for your plant depends on the quality of your incoming water. What may be right for one plant won’t necessarily be right for the same plant in a different geographic location. Great Lakes Water is considered surface water and makes up some of the purest water in Canada. This surface water typically “goes further” than ground water from an operating cost perspective as we can operate systems at higher cycles of concentration, blow down less and require less water treatment chemicals. Ground or well supplied water is typically higher in dissolved solids (calcium, magnesium, carbonates, bi carbonates, etc.). In short, not all water sources are created equally and boiler water treatment programs will vary depending on the source and quality of the feed water.
What’s Wrong with Alkalinity?
A quick review of ion exchange is required to understand dealkalization and we’ll use the water softening process as an example, as most boiler operators are very familiar with this. Water softeners use strong acid cation (SAC) resin for ion exchange. SAC resin has an affinity for divalent ions (Calcium, Magnesium) meaning that the resin wants to grab a hold of these divalent ions as they’re passing through the bed and exchange them with the sodium ions. Once resin is saturated and there are no more available free resin beads for ion exchange, a brute force wash of the SAC bead with sodium chloride (salt) brine is required.
Chloride Cycle Dealkalizer
Weak Acid Cation Dealkalizer
The same thing happens in the WAC resin bed. The H+ ion from the resin bead being in “hydrogen form” essentially destroys the carbonates and bicarbonates which are passing through the bed. The product of this reaction is water and carbon dioxide.
As soon as the hydrogen is relieved of duty having destroyed the carbonates/biocarbonates, this frees up the ion exchange site. The ion exchange site is “hungry” and the system is unbalanced. The WAC resin bead now has a negative ion exchange site open and we have calcium and magnesium (hardness) ions stranded with nowhere to go. The negative exchange sites want to take on something so they grab on to the calcium and magnesium.
Once the WAC bed has reached its capacity, the regeneration cycle is initiated once again. The resin bed is regenerated with acid. Since the WAC resin has an affinity for hydrogen and not calcium/magnesium, the calcium/magnesium ions readily wash off and are flushed down the drain as the H+ ion loosely binds to the resin bead again. The resin is now back in its hydrogen form and ready to go back into service.
So as we said before, the hydrogen is destroying the carbonate and bicarbonate and the product is water and carbon dioxide. The next step is we need to get rid of that carbon dioxide such that it doesn’t get to the boiler. This is accomplished with a degasifying tower and this is installed downstream of the WAC. The carbon dioxide and water are separated – the gaseous carbon dioxide is sent out to atmosphere while the water is sent to the next step.
Some important design considerations for the chloride cycle dealkalizer are:
- Feed water must be softened
- Calcium chloride can precipitate and foul the beads
- Minimal impact on total dissolved solids
- Potential small decrease in blowdown requirements
- Relatively low capital cost, reasonably effective, simple to operate
Some important design considerations for the WAC dealkalizer are:
- Additional softening required. WAC can remove as much hardness as there is available alkalinity – any residual hardness needs to be removed before the boiler.
- Efficiency reduction with increasing flow rate, decreasing kinetics.
- Handling of acid
- Sulfuric acid – heat of hydration is a concern (can’t have plastic tanks, plastic piping), higher concentrations are available (up to 93%), calcium sulfate precipitation can be a concern for water sources high in sulfate levels)
- Hydrochloric acid – fumes, plastic can be used, calcium chloride precipitation is not a concern, lower concentrations available (up to 32%)
- Higher capital cost, very effective, easy to operate, larger footprint
WAC Dealkalizer – Savings Breakdowns
For the WAC dealkalizer savings breakdown, both options result in an increase in COCs in the boiler and similar ROIs to the chloride cycle dealkalizer (approximately 6-7 years for great lakes water and 1 year for well water).
Again, not all water sources are created equally. Operating costs for a plant in Toronto will be very different for that same plant in rural Ontario which draws from a well. Neither dealkalizer option for the Great Lakes water steam plant offered a payback which would justify the project, but this was very different for the rural water customer. Whenever possible, we should strive to “remove rather than treat for” and in order to formulate a proper pre-treatment solution, a thorough understanding of incoming water quality is required.
Claire Beaureagard has a degree in Mechanical Engineering from The University of Western Ontario. As a Water & Energy Manager, she works with our clients to achieve the lowest operational costs for their heat transfer systems through well-maintained water treatment programs. Claire has a collection of unicycles she enjoys riding in her spare time.
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Water softening is a process in which water flows through a bed of resin to exchange the hardness ions, calcium and magnesium, for sodium ions. When the resin has reached its capacity for holding hardness ions, the